[Proj] How grids work in PROJ4

[Frank Warmerdam]

On Tue, Jan 10, 2012 at 3:49 PM, Michal Seidl <michal.seidl at gmail.com> wrote:
> Hello,
> I moved forward on implementing quite new Czech projection called "Krovak
> Modified" which is used in new ProjectedCRS EPSG:5224. I also want to
> implement vertical datum "Bpv" by use of +geoidgrids but I found myself
> lacking some basic information which I can not easily find  on the web.
>
> 1)  What kind of coordinate transformation is supported by PROJ 4.8.0?
>    - towgs84?
>    - nadgrids?
>    - geoidgrids?
>    - others?

Michal,

Yes, the above, and obviously projected transformations (ie proj=tmerc).

> 2) Can I combined these transformation? And how they are applied? Look at
> these situations:
>
> cs2cs ... +proj=latlong  +nadgrids=... +geoidgrids=...  +towgs84=...  +to
> +towgs84 ... +geoidgrids=... +nadgrids=...  +proj=latlong

Only one horizontal datum transformation can be associated with a
coordinate system.  So you should not currently mix +towgs84 and
+nadgrids.

> What happens?
>
> LLH->(dL,dL)->LLH->(dH)->LLH->XYZ->(3D)->XYZ(wgs84)->(3D)->XYZ->LLH->(dH)->LLH->(dL,dL)->LLH
>
> I mean by this:
> a) if there is +nadgrids  and +towgs84 on the LEFT SIDE PROJ firstly changes
> LL by dL,dL from grid before 3D transformation

Only one or the other is going to work properly.

> b) if there is +geoidgrids on the LEFT SIDE PROJ thinks there is orthometric
> height on input so firstly it changes H by dh from grid to get ellipsoid
> height before 3D transformation

Right

> c) on the RIGHT SITE it should work in a opposite order. Firstly do 3D
> transformation and than apply +nadgrids and +geoidgrids as there is
> ellipsoid height on the imput from 3D transformation

Right, with the caveat of only nadgrids or towgs84.

> I think PROJ should do this but I am not sure about it. I tried to do some
> test but I got confused. Is there some document how it exactly works?

Best regards,
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